1. Field of the Invention
This invention relates to a laser light output control apparatus for a semiconductor laser.
2. Description of the Related Art
A semiconductor laser is used to obtain a laser light by energizing with a driving current IF through a p-n junction of a certain type. As shown in FIG. 5, the relation between the driving current IF and a light output intensity PO is not linear, but as the driving current IF increases, laser light oscillation starts at a certain current value Ith, and hereafter, the laser light output PO increases with increasing the driving current IF.
However, a threshold current Ith and the rate of change (differential efficiency .eta.) of PO relative to the change of IF are not constant, but vary with the ambient temperature and also with individual lasers in which laser light oscillation takes place.
FIG. 6 is a block diagram of a laser light output control apparatus conventionally used to drive a semiconductor laser with a constant light output intensity.
Reference numeral 10 denotes an up/down counter which counts up an output value in accordance with an input clock signal when an output of a comparator 17 is at "H" level and then an UP/DOWN signal of the up/down counter 10 is at "H" level, and counts down the output value in accordance with the input clock signal when they are at "L" level.
The output value of the up/down counter 10 is converted by a D/A converter 12 into a laser driving current IF, and supplied through a current source 13 to a semiconductor laser 14 to drive the semiconductor laser 14. The light output PO of the laser 14 is converted by a monitor circuit 15 into a voltage, which is compared with a set voltage VM by a comparator 17.
The comparator 17 outputs a "H" level signal when the voltage from the monitor circuit 15 is lower than the set voltage VM and a "L" level signal when the voltage from the monitor circuit 15 is higher than the set voltage VM.
As described, the conventional laser light output control apparatus with the component parts mentioned above constitutes a negative feedback loop of the comparator 17. When control of the laser light output is started, the up/down counter 10 counts up, the output value of the D/A converter 12 and the laser driving current IF continue to increase and the laser light output increases gradually. After the monitor voltage obtained by monitoring the laser light exceeds the set voltage VM, the counter 10 counts up and down repeatedly, so that the light output PO comes to settle almost at a fixed value regulated by the set voltage VM.
FIG. 7 is a timing chart in a case where a laser light output control apparatus such as shown in FIG. 6 is used.
At the rising edge of a clock pulse (a) in FIG. 7, a counted-up value is output to the D/A converter 12 from the up/down counter 10, and accordingly, the driving current IF changes stepwise as illustrated in FIG. 7(b). When the driving current IF exceeds the threshold current Ith, the laser 14 starts oscillation, and increases the light output PO as shown in FIG. 7(c). As PO reaches to a target level, the counter 10 counts up and down at about that corresponding level, and therefore, IF fluctuates about a certain value, so that PO comes settle almost at the target level.
When the light output is to be set to some other value in the apparatus mentioned above, it is necessary to change the set voltage VM and start with counter actions. However, since the counter actions are performed by feeding the laser light back through the monitor circuit 15 and the comparator 17, a long time is always required for setting the laser light output.